Acoustic impact device

ABSTRACT

An impact device is described which is suitable for use in a precussive tool and which utilizes a hammer that impacts an anvil to efficiently transmit force pulses to a load such as the earth formation. The anvil system includes an impact hammer, an elastic spring member, drill steel and a bit. The required load force and load deflection are caused by a single force pulse of the proper force amplitude and time duration. The impact device elements and the load itself each have a mechanical impedance analogue which can be represented in a mechanical circuit. The Q of this circuit is selected to be between about 0.7 to 1.5 with the result that the transmitted force pulses are readily absorbed by the load with a minimum of reflection, while impact spring stress levels and velocities are both held to reasonable values. A folded spring configuration is described which permits the application of the present invention to relatively small pneumatic air tool devices.

United States Patent Primary Examiner-Ernest R. Purser Att0rneyMartin Lu Kacher ABSTRACT: An impact device is described which is suitable for use in a precussive tool and which utilizes a hammer that impacts an anvil to efficiently transmit force pulses to a load such as the earth formation. The anvil system includes an impact hammer, an elastic spring member, drill steel and a bit. The required load force and load deflection are caused by a single force pulse of the proper force amplitude and time duration. The impact device elements and the load itself each have a mechanical impedance analogue which can be represented in a mechanical circuit. The Q of this circuit is selected to be between about 0.7 to 1.5 with the result that the transmitted force pulses are readily absorbed by the load with a minimum of reflection, while impact spring stress levels and velocities are both held to reasonable values. A folded spring configuration is described which permits the application of the present invention to relatively small pneumatic air tool devices.

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MAXIM ATION PL ATTORNEY DEFLECTION PATENTEU 'm 1 6 I971 SHEET 3 0F 3 TYPICAL RANGE WITHOUT lMPACT SPRING PREFERRED RANGE WITH IMPACT SPRING 2 m. 0 0 m Q m m m s 2 II .I. 5 w T 2 m m 0 ...RH m m w P T 8 M w .0 H H n n w ,w 4. -2 0 1 s 2 TIME IN VENTOR. BOYD A. WISE ACQUSTIQ llMPACT DEVICE The present invention relates to impact devices and more particularly to impact or vibratory percussive tools.

The invention is especially suitable for use in hydroacoustic impact tools for earth boring and drilling applications. Other applications for the invention may be found in materials,

processing, machining, metal forming, pavement breakers and other applications which can more efficiently utilize mechanical energy impulses.

last impact tools have utilized a hammer element that impacts upon an anvil to couple energy to a load. Energy is wasted in the anvil system, which may include a drill steel and associated components, due to reflections of impulse energy which are not absorbed by the load in the drilling process This reflected energy may cause other objectional effects within the device.

Accordingly, it is an object of the present invention to provide an improved apparatus which is useful in percussive impact tools.

it is a further object of the present invention to provide an improved impact tool wherein energy is transferred to a load more efficiently than in previous tools of the type described.

it is a further object of the present invention to provide a relatively small impact tool wherein the above-mentioned difficulties and disadvantages are substantially eliminated.

It has been found that the load (i.e. earth formation) acts in part as a reactive element. In drilling earth formations, for example, the load provided by the formation effectively acts as a spring and presents a stiffness during the time interval that the drilling force is being applied. Although the overall or long term characteristic of an earth formation may include a resistive component, during the period that the penetration force is applied by the drilling bit to the formation, the formation has primarily a stiffness characteristic.

Utilizing this characteristic, an impact device embodying the present invention includes an impact hammer and an anvil system which may include a drill steel, bit means at one end of the drill steel and an elastic spring member disposed between the drill steel so as to be impacted by the hammer. These elements in combination with the earth formation or load present an equivalent circuit made up of mechanical impedance elements. Moreover, by adjusting the Q of this circuit, by varying the various materials and sizes of the above elements, to be within a range from about 0.7 to 1.5, load reflections are reduced thereby permitting impact energy to be efficiently transmitted to the load. In order to accommodate this invention in small devices, such as pneumatic air hammers, a compact folded spring configuration has been provided.

A feature of the present invention is that an impact device in accordance therewith may provide for transmission of impact pulses over longer distances than in arrangements heretofore.

Another feature of the invention is that in the elements of a device in accordance therewith, stress levels may be reduced with the exception of the impact spring which may be made of special materials adapted to withstand high-stress use.

A still further feature of the present invention is that devices in accordance therewith often -will have stress in their drill steel reduced by substantial amounts, often in the other order of about 50 percent.

A still further feature of the invention is to provide a folded spring configuration that can be used in a relatively small device.

The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will become more readily apparent from a reading of the following description in connection with the accompanying drawings in which:

P16. 1 is a simplified cross-sectional view of an impact tool embodying the invention;

H6. 2 is a typical load deflection curve for a wedge bit in a rock formation;

FZG. 3 is an enlarged fragmentary cross-sectional view of another tool embodying the invention but employing a folded impact spring;

FIG. 4 is a simplified mechanical analogue circuit of an impact tool in accordance with the present invention;

FIG. 5 is a graph plotting values of maximum normalized input force for different Q values forthe circuit of FIG. 4; and

FIG. 6 is a graph of force delivered to a load versus time for Q's equal to 0.2 and 1.0 for the circuit of FIG. 4 with each plot having an equal impact energy.

Referring to FIG. 1, an impact device in accordance with the invention includes a housing 10 that mounts a hammer 18 which is a massive cylindrical rod made of tough material such as alloy steel. The housing 10 has a bore portion in which an anvil system 24 is disposed. The anvil system 24 includes a sleeve 26, a guide member 30 and a bearing mechanism 32 located between the sleeve 26 and the bottom 30a of the guide member 30. The anvil system 24 also includes a hollow cylindrical shaft 34 known as the drill steel. The shaft 34 is made up of a number of sections .or subs which may be screwed together at joints, such as the joint 36. The shaft 34 is movable with respect to the sleeve 26. The upper end 340 of the shaft 34 is disposed adjacent to the bearing mechanism 32. The shaft 34 has a first web 38 across its inner periphery. Projections 40 extend from the shaft 34 in the region of the web 38 through openings 42 in the sleeve 26 and serve as stops to limit the downward excursion of the anvil.

The anvil system 24 also includes elastic spring 44 of material such as titanium alloy which is secured by a screw 45 at the lower end thereof to the web 38. A cap 46 of tough steel on the upper end of the spring 44 completes the anvil system 24. The spring 44 provides a compression spring for improving the transmission of impulse energy to the load, as will be explained more fully hereinafter.

Except during impact, the cap 46, provided at the upper end of the anvil system 24, is spaced from the lower end of the hammer element 18. The arrangement is such that the oscillatory motion of the hammer 18 is unimpeded, except during the fraction of the cycle when impact occurs. During impact a portion of such energy is imparted to the anvil system 24. This separation or decoupling of hammer l8 and anvil system 24 is further advantageous in that the oscillatory motion of the hammer is not apt to be stalled during initiation of oscillations; viz. the anvil, by virtue of being decoupled from the hammer, does not extract alternating energy from the hammer until oscillation is definitely established. For a complete description of a hammer and anvil system such as the one described, reference may be had to US. Pat. No. 3,382,932 issued to B. A. Wise on May 14,1968.

FIG. 3 shows an embodiment of an impact device 60 employing a folded impact spring 62 which may be used in a smaller impact device such as a pneumatic drill. The device 60 includes a hammer 64, folded spring 62 and an anvil system 68 (which includes a drill steel not shown). The hammer 64 is a cylindrical member which defines a cylindrical bore 70 into which is disposed the spring 62. The spring is fixedly secured by bolts 74 to the lower end of the hammer 64 and from this position extends upwardly into the bore 70 with an arm section 62a to a position near the top end of the bore 70. The arm 62a is welded onto arm 62b which extends back over the arm 62b to a position where it terminates in a block-shaped portion 78. The arm 6211, during an impact blow, is placed in tension while the arm 62b is in compression. The arms of course do not have to be disposed in the shown position and need not be of same cross-sectional areas.

An advantage of this arrangement is that the mass of the spring can be considered as part of the mass of the hammer so that, excluding the mass of the spring, a smaller hammer mass will be required. A further advantage is that the mass of the members between the hammer and spring which ordinarily would be the anvil surface and which ordinarily would have a mass concentration associated with it, is moving with the spring and does not act as an isolating element between the hammer and spring. It is also true that the forces on the anvil 68 are lower, and thus may be made smaller and will tend to allow a cross-sectional area transition from the impact system to the drill steel with a smaller discontinuity than would exist with the spring separable from the hammer.

Turning now to FIG. 2, the deflection characteristics of a typical rock formation, such as granite, limestone, marble or sandstone is shown. The penetration of the rock or load formation by a wedge-shaped tool is accomplished by means of a series of successive brittle fractures. For convenience of analysis the ascending line may be approximated by a straight line from the origin to a point of maximum load and deflection. A detailed description of this deflection characteristic is set forth in columns and 11 ofU.S. Pat. No. 3,382,932.

The energy absorbed for each blow corresponds to the areas under the curve which for convenience of analysis may be considered the triangle ABCA. Thus, from an analytical point of view, the load may be considered as a spring which during the period when energies are being absorbed has a stiffness:

KL F L) L 1 wherein K Load Stiffness F L Load Force P Load Penetration The load energy is:

As seen in FIG. 2, the energy absorbed by the load formation is delivered during the ascending portion of the force pulse and not during the descending trailing edge of this forcedeflection characteristic.

Turning now to FIG. 4, there is shown a simplified lumped constant equivalent mechanical impedance analogue circuit for either the device in FIG. 1 or FIG. 3 that includes spring compliance C a drill steel having the characteristic resistance R and the load compliance C the combination of which is in parallel with the hammer mass represented by a mass M. Mechanical switch 5-1 is in parallel with M; mechanical switch 8-2 is in parallel with C The hammer mass M develops a velocity V that is short-circuited by switch S-l until the time of impact. Switch S-l opens at the time of impact, requiring that the energy of the mass M to be coupled to the balance of the circuit.

The element C representing the impact spring, serves to control the rate of energy transfer and causes a pulse shape that will be absorbed by the load to a greater degree leading to improved energy transfer.

The element R, representing the drill steel, is treated in accordance with transmission line theory. For most purposes, losses are low and the drill steel may be considered as a lossless line with a resistive characteristic impedance and a propagation constant equal to its phase constant.

In most practical drilling applications, the drill steel is long enough so that it influences energy transfer from the hammer to the load and so has to be represented in the circuit. This is true for drill steels over about 2 feet in length; for the many applications in which several lengths of drill steel are used, the effect is multiplied in importance and tends to become dominant. For certain drilling applications involving down-hole tools, there is no drill steel as such. Even in this case, a properly designed impact spring will be desirable in reducing surface stress levels.

Drill steel characteristic impedance R is defined approximately by the combined product of the density, longitudinal velocity of sound and cross-sectional area of the uniform section of the drill steel between end couplings.

A more precise definition that takes into account nonuniform properties of the drill steel is that the drill steel impedance desired is the transfer impedance, which is the ratio of force to displacement, with force being applied at the pulse frequency and amplitude to the top end of the drill steel, and deflection being measured at the bottom end of the drill steel, when the drill steel is terminated so that no reflection occurs at the bottom end. It has been found unnecessary to use any such determination of the impedance in practice. Instead, the definition in the previous paragraph has sufiiced and the characteristic impedance taken from the weight per unit length of the drill steel and published values of density and velocity of sound in the drill steel material have sufficed.

The load compliance C, is the value determined by the ratio of the maximum load deflection to the maximum load force,

as detennined by a load deflection characteristic of the bits and rock combination, as given in FIG. 2. The value of interest corresponds to the ascending-force portion of the load-deflection characteristic. The descending-force portion of the loaddeflection characteristic may be adequately simulated by including the switch 8-2 that will be closed momentarily to dissipate the stored energy along the path B-C in FIG. 2. This artifice suffices since the small amount of energy actually returned to the system along the descending-force portion of the load-deflection characteristic is ordinarily not useful, but is dissipated by multiple round trips up and down the drill steel.

For a discussion of the derivation of mechanical circuits, reference may be had to the many texts which cover mechanical impedance analogues, such as for example, Mason, Electromechanical Transducers and Wave Filters, Van Nostrand, New York (1942); Olson, Dynamical Analogues, Van Nostrand, New York 1943).

Not all mechanical systems can be represented by lumpedconstant mechanical impedance analogue circuits. For the hammer to be a lumped mass M, it should have a negligible time delay in transmitting force at the pulse frequency from any significant element of its total mass to the pint of impact (usually the top of the impact spring). This requirement is sufficiently met if the time delay is less than one twenty-fourth of a complete cycle at the pulse frequency; the requirement is marginally met if the delay is one-twelfth of the period at the pulse frequency. For convenience in design, some small part of the hammer mass may be expected from meeting this requirement, and a significant element of the mass may be taken to be any mass element other than that one-tenth of the total mass that has the greatest time delay, although there is no discrete or absolute proportion of the mass that must be correctly lumped.

For a uniform cylindrical hammer with an axial impact velocity, the above hammer criteria are satisfied if the hammer has a length less than one twenty-fourth of a wavelength at the pulse frequency, or marginally has a length of a one-twelfth of a wavelength. A wavelength in the hammer is the ratio of the velocity of sound in the hammer to the pulse frequency.

For the impact spring to be a lumped compliance C it should have a negligible time delay in transmitting force at the pulse frequency from the force input surface of the spring to the force output surface of the spring. This requirement is sufficiently met if the time delay is less than one twenty-fourth of the period of a complete cycle at the pulse frequency; the requirement is marginally met if the delay is one-twelfth of a period at the pulse frequency. For a uniform cylindrical spring, loaded axially, it is equivalent to the above to specify that the spring have a length less than one twenty-fourth of a wavelength at the pulse frequency, or marginally, have a length of one-twelfth of a wavelength. A wavelength in the spring is the ratio of the velocity of sound in the spring to the pulse frequency.

From FIG. 4, it can be shown that if only a single force pulse is considered, the normalized input force may be found by folnwherein F force at top of drill steel 7 l Q W 1/ Q 1 W2 Q= RwC 2 11' times the ratio of stored energy to the energy dissipated per cycle e Base, Naperian logarithms R p (density) c( velocity of sound in drill steel) i S (cross-sectional area of drill steel) V= impact velocity F /R V= normalized input force n fraction of the repetitive pulse period during which impact occurs.

Using equation (3), FIG. 5 is a plot of the maximum force versus Q for equal values of impact energy with the same drill steel. The preferred range of operation for Q is shown (viz. 0.7 to 1.5).

FIG. 6 illustrates the normalized force delivered to the same load for circuit Qs of 0.2 (no spring) and 1.0 (impact spring) for equal impact energy. Comparison between these circuit Qs indicates that the impact spring (Q 1) has substantially reduced the stress in drill steel while at the same time increasing the force to the the load about 68 percent. From an energy point of view, the load energy caused by the first arrival of the force pulse is about 2.9 times as great when the impact spring is used.

The following equations, in addition to the above equations, can be used in designing the various sizes and selecting the elements comprising impact devices to obtain the proper circuit wherein m pulse frequency C; l/K spring compliance M hammer mass I length of impact spring 0, velocity of sound in the impact spring C l/K load compliance S cross-sectional area of drill steel 6 stress in psi.

The method of selecting impact system elements in accordance with the invention consists of a number of interrelated steps that must be performed in relation to one another. A logical sequence is indicated below that starts with a definition of the load requirements. If it were desired, an impact system could be designed first and it could then be determined what load to use it on or, equallypossible but less likely, a drill steel could be selected first, and then either the load or impact elements selected. The following logical sequence starts, as previously noted, with a definition of the load deflection for a midrange typical load. Thereafter, select a drill steel (material and cross-sectional) that will have a high but acceptable stress at the load force. A high stress level results in a lightweight drill steel and a low value of required static load, botla of which are usually desirable. Then calculate a combination of impact spring and hammer that will give the load deflection and load force using the selected steel.

For good efficiency of energy absorption, use a spring soft enough so that the Q of the circuit of FIG. 4 will have a value of about 0.7 or higher. Essentially all of the benefits of an impact spring are obtained at a Q value of 1.5 and it has been found that values higher than this tend to raise the stress level in the spring without commensurate gains in load energy absorption. Thereafter calculate the required impact velocity for the chosen hammer and load energy. Finally design the hammer drive (pneumatic, hydraulic, hydroacoustic, electric or other) to give the needed values of impact velocity, blow frequency and other parameters with the hammer selected.

Considering both the lower Q limit of about 0.7 and the upper limit about 1.5, a preferred range of Q for good design will be between 0.7 and 1.5. For most purposes, a Q of about 1.0 is a satisfactory selection.

While various embodiments of the invention have been described, variations thereof and modifications therein within the spirit of the invention will undoubtedly suggest themselves to those skilled in the art. Accordingly, the foregoing description should be taken as illustrative and not in any limiting sense.

I claim:

1. An impact device comprising:

a. a hammer element;

b. an anvil system having an impact spring member, a drill steel and means for transferring a force pulse from the hammer to the load; and c. the mass of said hammer element, the compliance of said impact spring member and the acoustic transmission of said drill steel having values such that the anvil system has a Q which is selected to be greater than about 0.7, wherein where C the compliance of said impact spring member M the mass of s said hammer element, and R p (density) c (velocity of sound in drill steel) S (cross-sectional area of drill steel).

2. The invention as set forth in claim 1 wherein said anvil system is configured so that it is represented by the following equivalent circuit:

a. said load represented by a compliance C b. said drill steel represented by transmission line theory as having card characteristic impedance R connected to the load;

c. said impact spring member represented by a lumped compliance C that shunts the top of said drill steel; and

d. said hammer element represented by a lumped mass M that shunts the top of the drill steel during impact.

3. The invention as set forth in claim 2 wherein said circuit includes all the elements and their circuit interrelationship shown in FIG. 4. g

4. The invention as set forth in claim 2 wherein said circuit Q is selected to be between 0.7 and 1.5.

5. The invention as set forth in claim 4 wherein said impact spring member is folded so that a portion thereof will be placed in tension and another portion in compression when it transfers the force pulse from said hammer to said drill steel.

6. The invention as set forth in claim 5 wherein said spring is a folded elastic spring member connected to said hammer.

7. An anvil system for a percussive tool having a hammer which comprises a drill steel adapted to have a bit connected to one end thereof, and a folded impact spring disposed between the opposite end of said drill steel and said hammer for transferring force pulses through said spring to said drill steel.

8. The invention as set forth in claim 7 wherein said spring is carried by said hammer.

9. The invention as set forth in claim 7 wherein said hammer has an opening at the end thereof facing said drill steel, and means for securing said folded spring within said opening, said spring having an outer section and an inner section, at least said inner section extending beyond said hammer face for impact with said steel. 

1. An impact device comprising: a. a hammer element; b. an anvil system having an impact spring member, a drill steel and means for transferring a force pulse from the hammer to the load; and c. the mass of said hammer element, the compliance of said impact spring member and the acoustic transmission of said drill steel having values such that the anvil system has a Q which is selected to be greater than about 0.7, wherein where CS the compliance of said impact spring member M the mass of s said hammer element, and R Rho (density) + OR - c (velocity of sound in drill steel) + OR - S (cross-sectional area of drill steel).
 2. The invention as set forth in claim 1 wherein said anvil system is configured so that it is represented by the following equivalent circuit: a. said load represented by a compliance CL; b. said drill steel represented by transmission line theory as having card characteristic impedance R connected to the load; c. said impact spring member represented by a lumped compliance CS that shunts the top of said drill steel; and d. said hammer element represented by a lumped mass M that shunts the top of the drill steel during impact.
 3. The invention as set forth in claim 2 wherein said circuit includes all the elements and their circuit interrelationship shown in FIG.
 4. 4. The invention as set forth in claim 2 wherein said circuit Q is selected to be between 0.7 and 1.5.
 5. The invention as set forth in claim 4 wherein said impact spring member is folded so that a portion thereof will be placed in tension and another portion in compression when it transfers the force pulse from said hammer to said drill steel.
 6. The invention as set forth in claim 5 wherein said spring is a folded elastic spring member connected to said hammer.
 7. An anvil system for a percussive tool having a hammer which comprises a drill steel adapted to have a bit connected to one end thereof, and a folded impact spring disposed between the opposite end of said drill steel and said hammer for transferring force pulses through said spring to said drill steel.
 8. The invention as set forth in claim 7 wherein said spring is carried by said hammer.
 9. The invention as set forth in claim 7 wherein said hammer has an opening at the end thereof facing said drill steel, and means for securing said folded spring within said opening, said spring having an outer section and an inner section, at least said inner section extending beyond said hammer face for impact with said steel. 